As a result of ever-increasing environmental obligations and cost pressure, there is at present a trend toward preparing PSAs with little if any solvent. This objective can most easily be realized by means of hotmelt technology. A further advantage is the reduction in production time. Hotmelt lines can laminate adhesives much more quickly to carriers or release paper, and so can save time and money.
However, the hotmelt technology always imposes stringent requirements on the adhesives. For high-grade industrial applications, particular preference is given to polyacrylates, on account of their transparency and stability to weathering.
In order to prepare acrylic hotmelts, conventionally, acrylic monomers are polymerized in solution and the solvent is then removed in the extruder in a concentrating process. Besides the advantages in transparency and weathering stability, however, acrylate PSAs are also required to meet exacting requirements in the shear strength field. This is achieved by means of polyacrylates of high molecular weight and high polarity with subsequent efficient crosslinking.
For the properties of PSAs, the orientation of the macromolecules also plays an important part. During the preparation, further processing, or subsequent (mechanical) stressing of polymers or polymer compositions, there may be high degrees of orientation of the macromolecules in preferential directions within the polymer assembly as a whole. This orientation may lead to special properties in the corresponding polymers. Some examples of properties which can be influenced by the degree of orientation include the strength and stiffness of the polymers and of the plastics produced from them, thermal conductivity, thermal stability, and anisotropy in respect of permeability to gases and liquids.
One important property dependent on the orientation of the monomer units is the refraction of light (expressed by the corresponding refractive index n) and the retardation δ.
From the standpoint of process engineering, advantages are afforded by electron beam crosslinking. For example, it is possible to “freeze in” certain states by the crosslinking process.
The retention of the partial orientation in partially crystalline rubber adhesives has already been described in U.S. Pat. No. 5,866,249. As a result of the anisotropic adhesive properties, innovative PSA applications were defined.
It is an object of the invention to provide oriented polyacrylate compositions which possess a sufficient lifetime of the anisotropic state and which by virtue of their orientation possess properties which are favorable in respect of their use as pressure sensitive adhesives, in particular in respect of their adhesive properties. A further object is to offer a process for preparing such pressure sensitive adhesives.